1. Field of the Invention
The present invention relates to a planetary gear assembly, and more particularly to a helical planetary gear assembly, for example, adapted for use in the power transfer device of a four-wheel drive vehicle.
2. Description of the Prior Art
In U.S. Pat. No. 4,440,042 issued on Apr. 3, 1984, there has been proposed a helical planetary gearz assembly which includes a housing, a ball bearing secured to the housing, an input shaft journalled in the ball bearing, an output shaft arranged coaxially with the input shaft and journalled in the housing for rotation relative to the input shaft, a sun gear splined to the input shaft for rotation therewith, a thrust washer interposed between the ball bearing and sun gear, a ring gear secured to the housing, a planet carrier free to float radially, and a plurality of planet gears supported by the carrier and being in meshing engagement with the sun and ring gears, all the gears having helical gear teeth. In this helical planetary gear assembly, thrust loads on the sun gear are absorbed by the housing through the thrust washer and ball bearing in one axial direction and through the input shaft in the other axial direction, thrust loads on the ring gear are absorbed directly by the housing, and the planet carrier is axially restrained by a locater plate between the sun gear and thrust washer and contiguous thereto such that thrust loads on the planet gears are absorbed by the housing through the locater plate, thrust washer and ball bearing or through the locater plate, sun gear and input shaft.
In the above arrangement, thrust loads on the planet gears will cause wear between the sun gear and locater plate during relative rotation thereof. For this reason, in the case that the helical planetary gear assembly is adapted to transfer large drive torque therethrough, there will occur durability problems in the sun gear and locater plate. Furthermore, in the above arrangement, the meshing engagement of the planet gears with the sun and ring gears will fluctuate due to radial displacement of the planet carrier in a speed reduction mode. It is, therefore, difficult to maintain the meshing engagement among the gears in a predetermined condition. This results in noise and vibration and in a decrease of power transmission efficiency.